Method and apparatus for transceiving channel status information in wireless communication system supporting cooperative transmission

ABSTRACT

The present invention relates to a wireless communication system. A method for reporting CSI (Channel State Information) in a cooperative multi-point (CoMP) wireless communication system, the method performed by a user equipment (UE) and comprising receiving first resource configuration information for a CSI-RS (Channel-State Information-Reference Signal) and second resource configuration information for interference measurement; and calculating CSI using the first resource configuration information and the second resource configuration information, the CSI being for one or more base stations (BSs) among a plurality of BSs participating the COMP, wherein an interference measurement resource according to the second resource configuration information exists in a union of zero-power CSI-RS resources of each of the plurality of BSs.

TECHNICAL FIELD

The present invention relates to a wireless communication system, andmore particularly, to a method and apparatus for calculating CSI(Channel State Information) using first resource configurationinformation for a CSI-RS (Channel-State Information-Reference Signal)and second resource configuration information for interferencemeasurement in a cooperative multi-point (CoMP) wireless communicationsystem. Especially, an interference measurement resource according tothe second resource configuration information exists in a union ofzero-power CSI-RS resources of each of the plurality of BSs.

Background Art

Multiple input multiple output (MIMO) increases the efficiency of datatransmission and reception using multiple transmit antennas and multiplereceive antennas instead of a single transmission antenna and a singlereception antenna. A receiver receives data through multiple paths whenmultiple antennas are used, whereas the receiver receives data through asingle antenna path when a single antenna is used. Accordingly, MIMO canincrease a data transmission rate and throughput and improve coverage.

A single cell MIMO scheme can be classified into a single user-MIMO(SU-MIMO) scheme for receiving a downlink signal by a single UE in onecell and a multi user-MIMO (MU-MIMO) scheme for receiving a downlinksignal by two or more UEs.

Research on cooperative multi-point (CoMP) for improving throughput of aUE located at a cell boundary by applying improved MIMO to a multi-cellenvironment is actively performed. The CoMP system can decreaseinter-cell interference in a multi-cell environment and improve systemperformance.

Channel estimation refers to a procedure for compensating for signaldistortion due to fading to restore a reception signal. Here, the fadingrefers to sudden fluctuation in signal intensity due to multipath-timedelay in a wireless communication system environment. For channelestimation, a reference signal (RS) known to both a transmitter and areceiver is required. In addition, the RS can be referred to as a RS ora pilot signal according to applied standard.

A downlink RS is a pilot signal for coherent demodulation for a physicaldownlink shared channel (PDSCH), a physical control format indicatorchannel (PCFICH), a physical hybrid indicator channel (PHICH), aphysical downlink control channel (PDCCH), etc. A downlink RS includes acommon RS (CRS) shared by all user equipments (UEs) in a cell and adedicated RS (DRS) for a specific UE. For a system (e.g., a systemhaving extended antenna configuration LTE-A standard for supporting 8transmission antennas) compared with a conventional communication system(e.g., a system according to LTE release-8 or 9) for supporting 4transmission antennas, DRS based data demodulation has been consideredfor effectively managing RSs and supporting a developed transmissionscheme. That is, for supporting data transmission through extendedantennas, DRS for two or more layers can be defined. DRS is pre-coded bythe same pre-coder as a pre-coder for data and thus a receiver caneasily estimate channel information for data demodulation withoutseparate precoding information.

A downlink receiver can acquire pre-coded channel information forextended antenna configuration through DRS but requires a separate RSother than DRS in order to non-pre-coded channel information.Accordingly, a receiver of a system according to LTE-A standard candefine a RS for acquisition of channel state information (CSI), that is,CSI-RS.

DISCLOSURE Technical Problem

An object of the present invention devised to solve the problem lies ina method and apparatus for reporting CSI in a cooperative multi-point(CoMP) wireless communication system.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

Technical Solution

The object of the present invention can be achieved by providing amethod for reporting CSI (Channel State Information) in a cooperativemulti-point (CoMP) wireless communication system, the method performedby a user equipment (UE) and including receiving first resourceconfiguration information for a CSI-RS (Channel-StateInformation-Reference Signal) and second resource configurationinformation for interference measurement; and calculating CSI using thefirst resource configuration information and the second resourceconfiguration information, the CSI being for one or more base stations(BSs) among a plurality of BSs participating the COMP, wherein ainterference measurement resource according to the second resourceconfiguration information exists in a union of zero-power CSI-RSresources of each of the plurality of BSs.

In another aspect of the present invention, provided herein is a methodfor receiving CSI (Channel State Information) in a cooperativemulti-point (CoMP) wireless communication system, the method performedby a base station (BS) and comprising: transmitting first resourceconfiguration information for a CSI-RS (Channel-StateInformation-Reference Signal) and second resource configurationinformation for interference measurement; and receiving CSI for one ormore BSs among a plurality of BSs participating the COMP, wherein ainterference measurement resource according to the second resourceconfiguration information exists in a union of zero-power CSI-RSresources of each of the plurality of BSs.

In another aspect of the present invention, provided herein is a userequipment (UE) for reporting CSI (Channel State Information) in acooperative multi-point (CoMP) wireless communication system, the UEcomprising: a radio frequency (RF) unit; and a processor, wherein theprocessor is configured to: receive first resource configurationinformation for a CSI-RS (Channel-State Information-Reference Signal)and second resource configuration information for interferencemeasurement; and calculate CSI using the first resource configurationinformation and the second resource configuration information, the CSIbeing for one or more base stations (BSs) among a plurality of BSsparticipating the COMP, and wherein a interference measurement resourceaccording to the second resource configuration information exists in aunion of zero-power CSI-RS resources of each of the plurality of BSs.

In another aspect of the present invention, provided herein is a basestation (BS) for receiving CSI (Channel State Information) in acooperative multi-point (CoMP) wireless communication system, the BScomprising: a radio frequency (RF) unit; and a processor, wherein theprocessor is configured to: transmit first resource configurationinformation for a CSI-RS (Channel-State Information-Reference Signal)and second resource configuration information for interferencemeasurement; and receive CSI for one or more BSs among a plurality ofBSs participating the COMP, and wherein a interference measurementresource according to the second resource configuration informationexists in a union of zero-power CSI-RS resources of each of theplurality of BSs.

The following features can be commonly applied to the embodiments of thepresent invention.

The method may further comprise receiving third resource configurationinformation for zero-power CSI-RS.

The third resource configuration information may include bitmapindicating a resource at which the zero-power CSI-RS is mapped.

The third resource configuration information may include subframeinformation indicating a subframe at which the zero-power CSI-RS istransmitted.

The second resource configuration information receiving may be receivedby using RRC (Radio Resource Control) signaling.

The CSI may include a CQI (Channel Quality Indicator).

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

Advantageous Effects

According to embodiments of the present invention, CSI can be moreeffectively transceived in a cooperative multi-point (CoMP) wirelesscommunication system.

In addition, according to the embodiments of the present invention, a UEcan calculate CSI (Channel State Information) using first resourceconfiguration information for a CSI-RS (Channel-StateInformation-Reference Signal) and second resource configurationinformation for interference measurement in a cooperative multi-point(CoMP) wireless communication system. Herein, an interferencemeasurement resource according to the second resource configurationinformation exists in a union of zero-power CSI-RS resources of each ofthe plurality of BSs.

It will be appreciated by persons skilled in the art that that theeffects that could be achieved with the present invention are notlimited to what has been particularly described hereinabove and otheradvantages of the present invention will be more clearly understood fromthe following detailed description taken in conjunction with theaccompanying drawings.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention.

In the drawings:

FIG. 1 illustrates the type-1 radio frame structure;

FIG. 2 illustrates the structure of a downlink resource grid for theduration of one downlink slot;

FIG. 3 illustrates the structure of a downlink subframe;

FIG. 4 illustrates the structure of an uplink subframe;

FIG. 5 illustrates the configuration of a MIMO communication systemhaving multiple antennas;

FIG. 6 illustrates a conventional CRS and DRS pattern.

FIG. 7 illustrates an exemplary DM RS pattern defined for the LTE-Asystem;

FIG. 8 illustrates exemplary CSI-RS patterns;

FIG. 9 is a diagram illustrating an example of a zero power (ZP) CSI-RSpattern;

FIG. 10 illustrates an example of CoMP;

FIG. 11 illustrates a case in which a DL CoMP operation is performed;

FIG. 12 is a flowchart illustrating a method for receiving dataaccording to a first embodiment of the present invention;

FIG. 13 is a diagram illustrating an example of EPDCCH according to anembodiment of the present invention; and

FIG. 14 is a diagram illustrating a BS and a UE to which an embodimentof the present invention can be applicable.

BEST MODE

The following embodiments are proposed by combining constituentcomponents and characteristics of the present invention according to apredetermined format. The individual constituent components orcharacteristics should be considered optional factors on the conditionthat there is no additional remark. If required, the individualconstituent components or characteristics may not be combined with othercomponents or characteristics. Also, some constituent components and/orcharacteristics may be combined to implement the embodiments of thepresent invention. The order of operations to be disclosed in theembodiments of the present invention may be changed. Some components orcharacteristics of any embodiment may also be included in otherembodiments, or may be replaced with those of the other embodiments asnecessary.

The embodiments of the present invention are disclosed on the basis of adata communication relationship between a base station and a terminal.In this case, the base station is used as a terminal node of a networkvia which the base station can directly communicate with the terminal.Specific operations to be conducted by the base station in the presentinvention may also be conducted by an upper node of the base station asnecessary.

In other words, it will be obvious to those skilled in the art thatvarious operations for enabling the base station to communicate with theterminal in a network composed of several network nodes including thebase station will be conducted by the base station or other networknodes other than the base station. The term “base station (BS)” may bereplaced with a fixed station, Node-B, eNode-B (eNB), or an access pointas necessary. The term “relay” may be replaced with the terms relay node(RN) or relay station (RS). The term “terminal” may also be replacedwith a user equipment (UE), a mobile station (MS), a mobile subscriberstation (MSS) or a subscriber station (SS) as necessary.

It should be noted that specific terms disclosed in the presentinvention are proposed for convenience of description and betterunderstanding of the present invention, and the use of these specificterms may be changed to other formats within the technical scope orspirit of the present invention.

In some instances, well-known structures and devices are omitted inorder to avoid obscuring the concepts of the present invention andimportant functions of the structures and devices are shown in blockdiagram form. The same reference numbers will be used throughout thedrawings to refer to the same or like parts.

Exemplary embodiments of the present invention are supported by standarddocuments disclosed for at least one of wireless access systemsincluding an institute of electrical and electronics engineers (IEEE)802 system, a 3rd generation partnership project (3GPP) system, a 3GPPlong term evolution (LTE) system, an LTE-advanced (LTE-A) system, and a3GPP2 system. In particular, steps or parts, which are not described toclearly reveal the technical idea of the present invention, in theembodiments of the present invention may be supported by the abovedocuments. All terminology used herein may be supported by at least oneof the above-mentioned documents.

The following embodiments of the present invention can be applied to avariety of wireless access technologies, for example, code divisionmultiple access (CDMA), frequency division multiple access (FDMA), timedivision multiple access (TDMA), orthogonal frequency division multipleaccess (OFDMA), single carrier frequency division multiple access(SC-FDMA), and the like. CDMA may be embodied through wireless (orradio) technology such as universal terrestrial radio access (UTRA) orCDMA2000. TDMA may be embodied through wireless (or radio) technologysuch as global system for mobile communication (GSM)/general packetradio service (GPRS)/enhanced data rates for GSM evolution (EDGE). OFDMAmay be embodied through wireless (or radio) technology such as instituteof electrical and electronics engineers (IEEE) 802.11 (Wi-Fi), IEEE802.16 (WiMAX), IEEE 802-20, and evolved UTRA (E-UTRA). UTRA is a partof universal mobile telecommunications system (UMTS). 3rd generationpartnership project (3GPP) long term evolution (LTE) is a part of E-UMTS(Evolved UMTS), which uses E-UTRA. 3GPP LTE employs OFDMA in downlinkand employs SC-FDMA in uplink. LTE—Advanced (LTE-A) is an evolvedversion of 3GPP LTE. WiMAX can be explained by IEEE 802.16e(wirelessMAN-OFDMA reference system) and advanced IEEE 802.16m(wirelessMAN-OFDMA advanced system). For clarity, the followingdescription focuses on IEEE 802.11 systems. However, technical featuresof the present invention are not limited thereto.

With reference to FIG. 1, the structure of a downlink radio frame willbe described below.

In a cellular orthogonal frequency division multiplexing (OFDM) wirelesspacket communication system, uplink and/or downlink data packets aretransmitted in subframes. One subframe is defined as a predeterminedtime period including a plurality of OFDM symbols. The 3GPP LTE standardsupports a type-1 radio frame structure applicable to frequency divisionduplex (FDD) and a type-2 radio frame structure applicable to timedivision duplex (TDD).

FIG. 1 illustrates the type-1 radio frame structure. A downlink radioframe is divided into 10 subframes. Each subframe is further dividedinto two slots in the time domain. A unit time during which one subframeis transmitted is defined as a Transmission Time Interval (TTI). Forexample, one subframe may be 1 ms in duration and one slot may be 0.5 msin duration. A slot includes a plurality of OFDM symbols in the timedomain and a plurality of resource blocks (RBs) in the frequency domain.Because the 3GPP LTE system adopts OFDMA for downlink, an OFDM symbolrepresents one symbol period. An OFDM symbol may be referred to as anSC-FDMA symbol or symbol period. An RB is a resource allocation unitincluding a plurality of contiguous subcarriers in a slot.

The number of OFDM symbols in one slot may vary depending on a cyclicprefix (CP) configuration. There are two types of CPs: extended CP andnormal CP. In the case of the normal CP, one slot includes 7 OFDMsymbols. In the case of the extended CP, the length of one OFDM symbolis increased and thus the number of OFDM symbols in a slot is smallerthan in the case of the normal CP. Thus when the extended CP is used,for example, 6 OFDM symbols may be included in one slot. If channelstate gets poor, for example, during fast movement of a UE, the extendedCP may be used to further decrease inter-symbol interference (ISI).

In the case of the normal CP, one subframe includes 14 OFDM symbolsbecause one slot includes 7 OFDM symbols. The first two or three OFDMsymbols of each subframe may be allocated to a physical downlink controlchannel (PDCCH) and the other OFDM symbols may be allocated to aphysical downlink shared channel (PDSCH).

The above-described radio frame structures are purely exemplary and thusit is to be noted that the number of subframes in a radio frame, thenumber of slots in a subframe, or the number of symbols in a slot mayvary.

FIG. 2 illustrates the structure of a downlink resource grid for theduration of one downlink slot. FIG. 2 corresponds to a case in which anOFDM includes normal CP. Referring to FIG. 2, a downlink slot includes aplurality of OFDM symbols in the time domain and includes a plurality ofRBs in the frequency domain. Here, one downlink slot includes 7 OFDMsymbols in the time domain and an RB includes 12 subcarriers in thefrequency domain, which does not limit the scope and spirit of thepresent invention. An element on a resource grid is referred to as aresource element (RE). For example, RE a(k,l) refers to RE location in ak_(th) subcarrier and a first OFDM symbol. In the case of the normal CP,one RB includes 12×7 REs (in the case of the extended CP, one RBincludes 12×6 REs). An interval between subcarriers is 15 kHz and thusone RB includes about 180 kHz in the frequency domain. N^(DL) is numberof RBs in a downlink slot. N^(DL) depends on a downlink transmissionbandwidth configured by BS scheduling.

FIG. 3 illustrates the structure of a downlink subframe. Up to threeOFDM symbols at the start of the first slot in a downlink subframe areused for a control region to which control channels are allocated andthe other OFDM symbols of the downlink subframe are used for a dataregion to which a PDSCH is allocated. A basic unit of transmission isone subframe. That is, a PDCCH and a PDSCH are allocated across twoslots. Downlink control channels used in the 3GPP LTE system include,for example, a physical control format indicator channel (PCFICH), aphysical downlink control channel (PDCCH), and a physical hybridautomatic repeat request (HARQ) indicator channel (PHICH). The PCFICH islocated in the first OFDM symbol of a subframe, carrying informationabout the number of OFDM symbols used for transmission of controlchannels in the subframe. The PHICH delivers an HARQACKnowledgment/Negative ACKnowledgment (ACK/NACK) signal in response toan uplink transmission. Control information carried on the PDCCH iscalled downlink control information (DCI). The DCI transports uplink ordownlink scheduling information, or uplink transmission power controlcommands for UE groups. The PDCCH delivers information about resourceallocation and a transport format for a downlink shared channel(DL-SCH), resource allocation information about an uplink shared channel(UL-SCH), paging information of a paging channel (PCH), systeminformation on the DL-SCH, information about resource allocation for ahigher-layer control message such as a random access responsetransmitted on the PDSCH, a set of transmission power control commandsfor individual UEs of a UE group, transmission power controlinformation, voice over Internet protocol (VoIP) activation information,etc. A plurality of PDCCHs may be transmitted in the control region. AUE may monitor a plurality of PDCCHs. A PDCCH is formed by aggregatingone or more consecutive Control Channel Elements (CCEs). A CCE is alogical allocation unit used to provide a PDCCH at a coding rate basedon the state of a radio channel. A CCE corresponds to a plurality of REgroups. The format of a PDCCH and the number of available bits for thePDCCH are determined according to the correlation between the number ofCCEs and a coding rate provided by the CCEs. An eNB determines the PDCCHformat according to DCI transmitted to a UE and adds a cyclic redundancycheck (CRC) to control information. The CRC is masked by an identifier(ID) known as a radio network temporary identifier (RNTI) according tothe owner or usage of the PDCCH. When the PDCCH is directed to aspecific UE, its CRC may be masked by a cell-RNTI (C-RNTI) of the UE.When the PDCCH is for a paging message, the CRC of the PDCCH may bemasked by a paging indicator identifier (P-RNTI). When the PDCCH carriessystem information, particularly, a system information block (SIB), itsCRC may be masked by a system information ID and a system informationRNTI (SI-RNTI). To indicate that the PDCCH carries a random accessresponse in response to a random access preamble transmitted by a UE,its CRC may be masked by a random access-RNTI (RA-RNTI).

FIG. 4 illustrates the structure of an uplink subframe. An uplinksubframe may be divided into a control region and a data region in thefrequency domain. A Physical Uplink Control Channel (PUCCH) carryinguplink control information is allocated to the control region and aphysical uplink shared channel (PUSCH) carrying user data is allocatedto the data region. To maintain the property of a single carrier, a UEdoes not transmit a PUSCH and a PUCCH simultaneously. A PUCCH for a UEis allocated to an RB pair in a subframe. The RBs of the RB pair occupydifferent subcarriers in two slots. Thus it is said that the RB pairallocated to the PUCCH is frequency-hopped over a slot boundary.

Modeling of MIMO System

A multiple input multiple output (MIMO) system increasestransmission/reception efficiency of data using multiple transmission(Tx) antennas and multiple reception (Rx) antennas. MIMO technology doesnot depend upon a single antenna path in order to receive all messagesbut instead can combine a plurality of data fragments received through aplurality of antennas and receive all data.

MIMO technology includes a spatial diversity scheme, a spatialmultiplexing scheme, etc. The spatial diversity scheme can increasetransmission reliability or can widen a cell diameter with diversitygain and thus is appropriate for data transmission of a UE that moves ahigh speed. The spatial multiplexing scheme can simultaneously transmitdifferent data so as to increase data transmission rate without increasein a system bandwidth.

FIG. 5 illustrates the configuration of a MIMO communication systemhaving multiple antennas. As illustrated in FIG. 5(a), the simultaneoususe of a plurality of antennas at both the transmitter and the receiverincreases a theoretical channel transmission capacity, compared to useof a plurality of antennas at only one of the transmitter and thereceiver. Therefore, transmission rate may be increased and frequencyefficiency may be remarkably increased. As channel transmission rate isincreased, transmission rate may be increased, in theory, to the productof a maximum transmission rate R_(o) that may be achieved with a singleantenna and a transmission rate increase Ri.

R _(i)=min(N _(T) ,N _(R))  [Equation 1]

For instance, a MIMO communication system with four Tx antennas and fourRx antennas may achieve a four-fold increase in transmission ratetheoretically, relative to a single-antenna system. Since thetheoretical capacity increase of the MIMO system was verified in themiddle 1990s, many techniques have been actively proposed to increasedata rate in real implementation. Some of the techniques have alreadybeen reflected in various wireless communication standards for 3G mobilecommunications, future-generation wireless local area network (WLAN),etc.

Concerning the research trend of MIMO up to now, active studies areunderway in many respects of MIMO, inclusive of studies of informationtheory related to calculation of multi-antenna communication capacity indiverse channel environments and multiple access environments, studiesof measuring MIMO radio channels and MIMO modeling, studies oftime-space signal processing techniques to increase transmissionreliability and transmission rate, etc.

Communication in a MIMO system will be described in detail throughmathematical modeling. It is assumed that N_(T) Tx antennas and N_(R) Rxantennas are present in the system.

Regarding a transmission signal, up to N_(T) pieces of information canbe transmitted through the N_(T) Tx antennas, as expressed in Equation 2below.

s=└s ₁ ,s ₂ , . . . s _(n) _(T)   [Equation 2]

A different transmission power may be applied to each piece oftransmission information, s₁, s₂, . . . , s_(N) _(T) . Let thetransmission power levels of the transmission information be denoted byP₁, P₂, . . . , P_(N) _(T) , respectively. Then the transmissionpower-controlled transmission information vector is given as

ŝ=[ŝ ₁ ,ŝ ₂ , . . . ,ŝ _(N) _(T) ]^(T) =[P ₁ s ¹ ,P ₂ s ₂ , . . . ,P_(N) _(T) s _(N) _(T) ]^(T)  [Equation 3]

The transmission power-controlled transmission information vector s maybe expressed as follows, using a diagonal matrix P of transmissionpower.

$\begin{matrix}{\hat{s} = {{\begin{bmatrix}P_{1} & \; & \; & 0 \\\; & P_{2} & \; & \; \\\; & \; & \ddots & \; \\0 & \; & \; & P_{N_{T}}\end{bmatrix}\begin{bmatrix}s_{1} \\s_{2} \\\vdots \\s_{N_{T}}\end{bmatrix}} = {Ps}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

N_(T) transmission signals x₁, x₂, . . . , x_(N) _(T) may be generatedby multiplying the transmission power-controlled information vector ŝ bya weight matrix W. The weight matrix W functions to appropriatelydistribute the transmission information to the Tx antennas according totransmission channel states, etc. These N_(T) transmission signals x₁,x₂, . . . , x_(N) _(T) are represented as a vector x, which may bedetermined by Equation 5 below.

$\begin{matrix}{x = {\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{i} \\\vdots \\x_{N_{T}}\end{bmatrix} = {\quad{{\begin{bmatrix}w_{11} & w_{12} & \ldots & w_{1N_{T}} \\w_{21} & w_{22} & \ldots & w_{2N_{T}} \\\vdots & \; & \ddots & \; \\w_{i\; 1} & w_{i\; 2} & \ldots & w_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\w_{N_{T}1} & w_{N_{T}2} & \ldots & w_{N_{T}N_{T}}\end{bmatrix}\begin{bmatrix}{\hat{s}}_{1} \\{\hat{s}}_{2} \\\vdots \\{\hat{s}}_{j} \\\vdots \\{\hat{s}}_{N_{T}}\end{bmatrix}} = {{W\hat{s}} = {WPs}}}}}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

Here, w_(ij) refers to a weight between an i_(th) Tx antenna and j_(th)information.

A reception signal x may be considered in different ways according totwo cases (e.g., spatial diversity and spatial multiplexing). In thecase of spatial multiplexing, different signals are multiplexed and themultiplexed signals are transmitted to a receiver, and thus, elements ofinformation vector (s) have different values. In the case of spatialdiversity, the same signal is repeatedly transmitted through a pluralityof channel paths and thus elements of information vectors (s) have thesame value. A hybrid scheme of spatial multiplexing and spatialdiversity can also be considered. That is, that same signal may betransmitted through three Tx antennas and the remaining signals may bespatial-multiplexed and transmitted to a receiver.

In the case of N_(R) Rx antennas, a reception signal of each antenna maybe expressed as the vector shown in Equation 6 below.

y=[y ₁ ,y ₂ , . . . ,y _(N) _(R) ]^(T)  [Equation 6]

When a channel modeling is executed in the MIMO communication system,individual channels can be distinguished from each other according totransmission/reception (Tx/Rx) antenna indexes. A channel passing therange from a Tx antenna j to an Rx antenna i is denoted by h_(ij). Itshould be noted that the index order of the channel h_(ij) is locatedbefore a reception (Rx) antenna index and is located after atransmission (Tx) antenna index.

FIG. 5(b) illustrates channels from N_(T) Tx antennas to an Rx antennai. The channels may be collectively represented in the form of vectorand matrix. Referring to FIG. 5(b), the channels passing the range fromthe N_(T) Tx antennas to the Rx antenna i can be represented by theEquation 7 below.

h _(i) ^(T) =[h _(i1) ,h _(i2) , . . . ,h _(iN) _(T) ]  [Equation 7]

All channels passing the range from the N_(T) Tx antennas to N_(R) Rxantennas are denoted by the matrix shown in Equation 8 below.

$\begin{matrix}{H = {\begin{bmatrix}h_{1}^{T} \\h_{2}^{T} \\\vdots \\h_{i}^{T} \\\vdots \\h_{N_{R}}^{T}\end{bmatrix} = \begin{bmatrix}h_{11} & h_{12} & \ldots & h_{1N_{T}} \\h_{21} & h_{22} & \ldots & h_{2N_{T}} \\\vdots & \; & \ddots & \; \\h_{i\; 1} & h_{i\; 2} & \ldots & h_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\h_{N_{R}1} & h_{N_{R}2} & \ldots & h_{N_{R}N_{T}}\end{bmatrix}}} & \left\lbrack {{Equation}\mspace{14mu} 8} \right\rbrack\end{matrix}$

Additive white Gaussian noise (AWGN) is added to an actual channel whichhas passed the channel matrix. The AWGN (n₁, n₂, . . . , n_(NR)) addedto each of N_(R) reception (Rx) antennas can be represented by Equation9 below.

n=[n ₁ ,n ₂ , . . . ,n _(N) _(R) ]^(T)  [Equation 9]

A reception signal calculated by the above-mentioned equations can berepresented by Equation 10 below.

$\begin{matrix}{y = {\begin{bmatrix}y_{1} \\y_{2} \\\vdots \\y_{i} \\\vdots \\y_{N_{R}}\end{bmatrix} = {\quad{{\begin{bmatrix}h_{11} & h_{12} & \ldots & h_{1N_{T}} \\h_{21} & h_{22} & \ldots & h_{2N_{T}} \\\vdots & \; & \ddots & \; \\h_{i\; 1} & h_{i\; 2} & \ldots & h_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\h_{N_{R}1} & h_{N_{R}2} & \ldots & h_{N_{R}N_{T}}\end{bmatrix}\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{j} \\\vdots \\x_{N_{T}}\end{bmatrix}} + \begin{bmatrix}n_{1} \\n_{2} \\\vdots \\n_{i} \\\vdots \\n_{N_{R}}\end{bmatrix}}}}} & \left\lbrack {{Equation}\mspace{14mu} 10} \right\rbrack\end{matrix}$

The number of rows and the number of columns of a channel matrix Hindicating a channel condition are determined by the number of Tx/Rxantennas. In the channel matrix H, the number of rows is equal to thenumber (N_(R)) of Rx antennas, and the number of columns is equal to thenumber (N_(T)) of Tx antennas. Namely, the channel matrix H is denotedby an N_(R)×N_(T) matrix.

The rank of a matrix is defined as the smaller between the number ofindependent rows and the number of independent columns in the channelmatrix. Accordingly, the rank of the channel matrix is not larger thanthe number of rows or columns of the channel matrix. The rank of achannel matrix H, rank(H) satisfies the following constraint.

rank(H)≦min(N _(T) ,N _(R)  [Equation 11]

For MIMO transmission, ‘rank’ indicates the number of paths forindependent transmission of signals and ‘number of layers’ indicates thenumber of streams transmitted through each path. In general, atransmission end transmits layers, the number of which corresponds tothe number of ranks used for signal transmission, and thus, rank havethe same meaning as number of layers unless there is no differentdisclosure.

Reference Signals (RSs)

In a wireless communication system, a packet is transmitted on a radiochannel. In view of the nature of the radio channel, the packet may bedistorted during the transmission. To receive the signal successfully, areceiver should compensate for the distortion of the reception signalusing channel information. Generally, to enable the receiver to acquirethe channel information, a transmitter transmits a signal known to boththe transmitter and the receiver and the receiver acquires knowledge ofchannel information based on the distortion of the signal received onthe radio channel. This signal is called a pilot signal or an RS.

In the case of data transmission and reception through multipleantennas, knowledge of channel states between transmission (Tx) antennasand reception (Rx) antennas is required for successful signal reception.Accordingly, an RS should be transmitted through each Tx antenna.

RSs in a mobile communication system may be divided into two typesaccording to their purposes: RS for channel information acquisition andRS for data demodulation. Since its purpose lies in that a UE acquiresdownlink channel information, the former should be transmitted in abroad band and received and measured even by a UE that does not receivedownlink data in a specific subframe. This RS is also used in asituation like handover. The latter is an RS that an eNB transmits alongwith downlink data in specific resources. A UE can estimate a channel byreceiving the RS and accordingly can demodulate data. The RS should betransmitted in a data transmission area.

A legacy 3GPP LTE (e.g., 3GPP LTE release-8) system defines two types ofdownlink RSs for unicast services: a common RS (CRS) and a dedicated RS(DRS). The CRS is used for acquisition of information about a channelstate, measurement of handover, etc. and may be referred to as acell-specific RS. The DRS is used for data demodulation and may bereferred to as a UE-specific RS. In a legacy 3GPP LTE system, the DRS isused for data demodulation only and the CRS can be used for bothpurposes of channel information acquisition and data demodulation.

CRSs, which are cell-specific, are transmitted across a wideband inevery subframe. According to the number of Tx antennas at an eNB, theeNB may transmit CRSs for up to four antenna ports. For instance, an eNBwith two Tx antennas transmits CRSs for antenna port 0 and antennaport 1. If the eNB has four Tx antennas, it transmits CRSs forrespective four Tx antenna ports, antenna port 0 to antenna port 3.

FIG. 6 illustrates a CRS and DRS pattern for an RB (including 14 OFDMsymbols in time by 12 subcarriers in frequency in case of a normal CP)in a system where an eNB has four Tx antennas. In FIG. 6, REs labeledwith ‘R0’, ‘R1’, ‘R2’ and ‘R3’ represent the positions of CRSs forantenna port 0 to antenna port 4, respectively. REs labeled with ‘D’represent the positions of DRSs defined in the LTE system.

The LTE-A system, an evolution of the LTE system, can support up toeight Tx antennas. Therefore, it should also support RSs for up to eightTx antennas. Because downlink RSs are defined only for up to four Txantennas in the LTE system, RSs should be additionally defined for fiveto eight Tx antenna ports, when an eNB has five to eight downlink Txantennas in the LTE-A system. Both RSs for channel measurement and RSsfor data demodulation should be considered for up to eight Tx antennaports.

One of significant considerations for design of the LTE-A system isbackward compatibility. Backward compatibility is a feature thatguarantees a legacy LTE terminal to operate normally even in the LTE-Asystem. If RSs for up to eight Tx antenna ports are added to atime-frequency area in which CRSs defined by the LTE standard aretransmitted across a total frequency band in every subframe, RS overheadbecomes huge. Therefore, new RSs should be designed for up to eightantenna ports in such a manner that RS overhead is reduced.

Largely, new two types of RSs are introduced to the LTE-A system. Onetype is CSI-RS serving the purpose of channel measurement for selectionof a transmission rank, a modulation and coding scheme (MCS), aprecoding matrix index (PMI), etc. The other type is demodulation RS (DMRS) for demodulation of data transmitted through up to eight Txantennas.

Compared to the CRS used for both purposes of measurement such aschannel measurement and measurement for handover and data demodulationin the legacy LTE system, the CSI-RS is designed mainly for channelestimation, although it may also be used for measurement for handover.Since CSI-RSs are transmitted only for the purpose of acquisition ofchannel information, they may not be transmitted in every subframe,unlike CRSs in the legacy LTE system. Accordingly, CSI-RSs may beconfigured so as to be transmitted intermittently (e.g. periodically)along the time axis, for reduction of CSI-RS overhead.

When data is transmitted in a downlink subframe, DM RSs are alsotransmitted dedicatedly to a UE for which the data transmission isscheduled. Thus, DM RSs dedicated to a particular UE may be designedsuch that they are transmitted only in a resource area scheduled for theparticular UE, that is, only in a time-frequency area carrying data forthe particular UE.

FIG. 7 illustrates an exemplary DM RS pattern defined for the LTE-Asystem. In FIG. 7, the positions of REs carrying DM RSs in an RBcarrying downlink data (an RB having 14 OFDM symbols in time by 12subcarriers in frequency in case of a normal CP) are marked. DM RSs maybe transmitted for additionally defined four antenna ports, antenna port7 to antenna port 10 in the LTE-A system. DM RSs for different antennaports may be identified by their different frequency resources(subcarriers) and/or different time resources (OFDM symbols). This meansthat the DM RSs may be multiplexed in frequency division multiplexing(FDM) and/or time division multiplexing (TDM). If DM RSs for differentantenna ports are positioned in the same time-frequency resources, theymay be identified by their different orthogonal codes. That is, these DMRSs may be multiplexed in Code Division Multiplexing (CDM). In theillustrated case of FIG. 7, DM RSs for antenna port 7 and antenna port 8may be located on REs of DM RS CDM group 1 through multiplexing based onorthogonal codes. Similarly, DM RSs for antenna port 9 and antenna port10 may be located on REs of DM RS CDM group 2 through multiplexing basedon orthogonal codes.

FIG. 8 illustrates exemplary CSI-RS patterns defined for the LTE-Asystem. In FIG. 8, the positions of REs carrying CSI-RSs in an RBcarrying downlink data (an RB having 14 OFDM symbols in time by 12subcarriers in frequency in case of a normal CP) are marked. One of theCSI-RS patterns illustrated in FIGS. 8(a) to 8(e) is available for anydownlink subframe. CSI-RSs may be transmitted for eight antenna portssupported by the LTE-A system, antenna port 15 to antenna port 22.CSI-RSs for different antenna ports may be identified by their differentfrequency resources (subcarriers) and/or different time resources (OFDMsymbols). This means that the CSI-RSs may be multiplexed in FDM and/orTDM. CSI-RSs positioned in the same time-frequency resources fordifferent antenna ports may be identified by their different orthogonalcodes. That is, these DM RSs may be multiplexed in CDM. In theillustrated case of FIG. 8(a), CSI-RSs for antenna port 15 and antennaport 16 may be located on REs of CSI-RS CDM group 1 through multiplexingbased on orthogonal codes. CSI-RSs for antenna port 17 and antenna port18 may be located on REs of CSI-RS CDM group 2 through multiplexingbased on orthogonal codes. CSI-RSs for antenna port 19 and antenna port20 may be located on REs of CSI-RS CDM group 3 through multiplexingbased on orthogonal codes. CSI-RSs for antenna port 21 and antenna port22 may be located on REs of CSI-RS CDM group 4 through multiplexingbased on orthogonal codes. The same principle described with referenceto FIG. 8(a) is applicable to the CSI-RS patterns illustrated in FIGS.8(b) to 8(e).

FIG. 9 is a diagram illustrating an example of a zero power (ZP) CSI-RSpattern defined in an LTE-A system. The use of the ZP CSI-RS can belargely classified into two types. A first type of ZP CSI-RS is forenhancing CSI-RS performance. That is, one network may performs mutingon CSI-RS RE of another network in order to enhance the measurementperformance of CSI-RS of another network and may configure and signalthe muted RE as ZP CSI-RS such that a UE of the network mayappropriately perform rate matching. A second type of ZP CSI-RS is forinterference measurement for calculation of CoMP CQI. That is, somenetworks may perform muting on ZP CRS-RS RE, and the UE may measureinterference from ZP CSI-RS to calculate CoMP CQI.

RS patterns of FIGS. 6 to 9 are purely exemplary. Various embodiments ofthe present invention are not limited to specific RS patterns. That is,when different RS patterns from in FIGS. 6 to 9 are defined and used,the various embodiments of the present invention may be applied in thesame way.

CSI Feedback of Cooperative Multipoint Transmission/Reception (CoMP)System

Hereinafter, CoMP will be described.

A post LTE-A system tries to use a method for allowing cooperationbetween plural cells to enhance system performance. This method isreferred to as cooperative multipoint transmission/reception (CoMP).CoMP refers to a scheme in which two or more BSs, access points, orcells communicate with a UE in cooperation with each other for smoothcommunication between a BS, an access point, or a cell with a specificUE. According to the present invention, a BS, an access point, and acell may be used in the same meaning.

It is known that Inter-Cell Interference (ICI) generally degrades theperformance of a UE at a cell edge and average sector throughput in amulti-cellular environment with a frequency reuse factor of 1. To offeran appropriate throughput performance to a cell-edge UE in anenvironment constrained by interference, a simple ICI mitigationtechnique such as UE-specific power control-based fractional frequencyreuse (FFR) is used in the conventional LTE system. However, it may bepreferred to reduce the ICI or reuse the ICI as a desired signal for theUE, rather than to decrease the utilization of frequency resources percell. For this purpose, CoMP transmission techniques may be adopted.

FIG. 10 illustrates an example of CoMP. Referring to FIG. 10, a wirelesscommunication system includes a plurality of BSs BS1, BS2, and BS3 whichperform CoMP and a UE. The plural BSs BS1, BS2, and BS3 which performCoMP may effectively transmit data to the UE in cooperation with eachother.

A CoMP transmission scheme may be classified into CoMP joint processing(JP) via data sharing and CoMP-cooperative scheduling/beamforming(CS/CB).

According to CoMP-JP applicable to downlink, a UE may simultaneouslyreceive data from a plurality of BSs that perform a CoMP transmissionscheme and may combine signals received from the BSs to enhancereception performance (joint transmission; JT). In addition, one of BSsthat perform a CoMP transmission scheme may transmit data to the UE at aspecific point of time (Dynamic point selection; DPS). According toCoMP-CS/CB, the UE may momentarily receive data from one BS, that is, aserving BS via beamforming.

When CoMP-JP is applied to uplink, a plurality of BSs may simultaneouslyreceive a PUSCH signal from a BS (Joint Reception; JR). On the otherhand, in case of CoMP-CS/CB, only one BS may receive a PUSCH.Cooperative cells (or BSs) may determine to use cooperativescheduling/beamforming (CS/CB).

A UE using a CoMP transmission scheme, that is, a CoMP UE may transmitchannel information as feedback (hereinafter, referred to as CSIfeedback) to a plurality of BSs that perform a CoMP transmission scheme.A network scheduler may select an appropriate CoMP transmission schemefor increasing a transmission rate among CoMP-JP, CoMP-CS/CB, and DPSmethods, based on the CSI feedback. To this end, a CoMP UE may configurethe CSI feedback in a plurality of BSs that perform a CoMP transmissionscheme according to a periodic feedback transmission scheme using ULPUCCH. In this case, feedback configuration of each BS may beindependent from each other. Thus, hereinafter, in this specification,according to an embodiment of the present invention, an operation fortransmitting channel information as feedback with independent feedbackconfiguration is referred to as a CSI process. One or more CSI processesmay be present in one serving cell.

FIG. 11 illustrates a case in which a DL CoMP operation is performed.

In FIG. 11, a UE is positioned between eNB1 and eNB2. The two eNBs(i.e., eNB1 and eNB2) perform a CoMP operation such as JT, DCS, andCS/CB in order to overcome interference with the UE. The UE performsappropriate CSI feedback for facilitating the CoMP operation of an eNB.Information transmitted via CSI feedback may include PMI information ofeach eNB and CQI information and may further include channel information(e.g., phase offset information between the two eNB channels) betweenthe two eNBs for JT.

Although FIG. 11 illustrates a case in which the UE transmits a CSIfeedback signal to eNB1 that is a serving cell of the UE, the UE maytransmit the CSI feedback signal to eNB2 or the two eNBs according to asituation. In addition, although FIG. 11 illustrates a case in which abasic unit participating in CoMP is eNB, the present invention may beapplied to CoMP between transmission points controlled by the eNB.

For CoMP scheduling in a network, the UE needs to feedback DL CSIinformation of neighboring eNB that participates in CoMP as well DL CSIinformation of serving eNB. To this end, the UE may feedback a pluralityof CSI processes that reflect various data transmission eNB and variousinterference environments.

Thus, an LTE system uses an interference measurement resource (IMR) forinterference measurement during calculation of CoMP CSI. One UE may beconfigured by a plurality of IMRs which have independent configuration.That is, the IMRs may be configured by independent periods, offsets, andresource configuration, and a BS may signal IMR to a UE via higher-layersignaling (RRC, etc.).

In addition, an LTE system uses CSI-RS in order to measure a channeldesired for calculation of CoMP CSI. One UE may be configured by aplurality of CSI-RSs which have independent configurations. That is,each CSI-RS may be configured by independent periods, offsets, resourceconfiguration, power control (Pc), and number of antenna ports. CSI-RSrelated information may be transmitted to a UE from a BS viahigher-layer signaling (RRC, etc.).

Among a plurality of CSI-RSs and a plurality of IMRs configured to theUE, one CSI process may be defined in association with one CSI-RSresource for signal measurement and one interference measurementresource (IMR) for interference measurement. The UE feedbacks CSIinformation obtained via different CSI processes with independentperiods and subframe offsets.

That is, each CSI process has independent CSI feedback configurations.The CSI-RS resource, the IMR resource association information, and theCSI feedback configuration may be indicated to the UE by a BS viahigher-layer signaling for each respective CSI process. For example, itis assumed that the UE may be configured by three CSI processes shown inTable 1 below.

TABLE 1 Signal Measurement CSI Process Resource (SMR) IMR CSI process 0CSI-RS 0 IMR 0 CSI process 1 CSI-RS 1 IMR 1 CSI process 2 CSI-RS 0 IMR 2

In Table 1 above, CSI-RS 0 and CSI-RS 1 are CSI-RS received from eNB 1that is a serving eNB of the UE and CSI-RS received from eNB 2 as aneighboring eNB that participates in cooperation, respectively. When itis assumed that IMR configured for each respective CSI process of Table1 above is configured as shown in Table 2 below,

TABLE 2 IMR eNB 1 eNB 2 IMR 0 Muting Data transmission IMR 1 Datatransmission Muting IMR 2 Muting Muting

With regard to IMR 0, eNB 1 performs muting and eNB 2 performs datatransmission, and the UE is configured to measure interference from eNBsexcept for eNB 1 based on IMR 0. Similarly, with regard to IMR 1, eNB 2performs muting and eNB 1 performs data transmission, and the UE isconfigured to measure interference from eNBs except for eNB 2 based onIMR 1. In addition, with regard to IMR 2, both eNB 1 and eNB 2 performmuting, and the UE is configured to measure interference from eNBsexcept for eNB 1 and eNB 2 based on IMR 2.

Accordingly, as shown in Tables 1 and 2 above, CSI information of CSIprocess 0 refers to optimum RI, PMI, and CQI information when data isreceived from eNB 1. CSI information of CSI process 1 refers to optimumRI, PMI, and CQI when data is received from eNB 2. CSI information ofCSI process 2 refers to optimum RI, PMI, and CQI information when datais received from eNB 1 and interference is not generated from eNB 2.

All IMRs configured for one UE may be indicated by zero power (ZP)CSI-RS. That is, the UE assumes that data of the UE is not mapped in theconfigured IMR and performs PDSCH rate matching during data reception.

Here, all IMRs are indicated by ZP CSI-RS because a CoMP UE cannot knoweNB from which data is actually received. For example, in FIG. 10,during DPS CoMP, the UE does not know eNB among eNB 1 and eNB 2, fromwhich data is actually transmitted, and receives data without separatesignaling.

When eNB 1 transmits data and the UE knows the fact, IMR 1 may be usedto receive data as well as to measure interference. On the other hand,when eNB 2 transmits data and the UE knows the fact, IMR 0 may be usedto receive data as well as to measure interference. However, when the UEdoes not know eNB that transmits data, it is effective to assume mutingwith respect to IMR 0 and IMR 1 and perform PDSCH rate matching in orderto reduce decoding errors.

According to a method in which all IMRs are indicated by ZP CSI-RS, theUE cannot receive data with respect to all configured IMRs, problemsarise in that PDSCH resources are wasted. This is because that UEassumes that data is not transmitted from all configured IMRs andperforms PDSCH rate matching.

First Embodiment

Hereinafter, an embodiment of the present invention in which a UE alsoreceives data from configured IMR and more effectively uses PDSCHresource will be described. To this end, the UE receives ZP CSI-RSinformation for muting per eNB and transmission eNB informationindicating eNB that actually transmits data.

The ZP CSI-RS information for muting per eNB may be transmitted to theUE by higher-layer signaling such as RRC signaling, etc. For example, inFIG. 11, eNB 1 performs muting in IMR 0 and IMR 2, and thus, the UEreceives ZP CSI-RS information of eNB 1 including IMR 0 and IMR 2. Onthe other hand, eNB 2 performs muting in IMR 1 and IMR 2, and thus, theUE receives ZP CSI-RS information of eNB 2 including IMR 1 and IMR 2.

ZP CSI-RS information of each eNB includes a period of ZP CSI-RS,subframe offset, and resource configuration. These values may beindependently configured for each ZP CSI-RS of each eNB but ZP CSI-RS ofeach eNB may be limited to have the same period and subframe offset inorder to minimize impact on a legacy UE. Through this limitation, thenumber of subframes, ZP CSI-RS of which is not configured, may beincreased and a BS may schedule the legacy UE to the subframe tominimize data decoding error due to data mapping mismatch.

Transmission eNB information indicating eNB that actually transmits datamay be dynamically transmitted to the UE from the eNB through DCI inPDCCH. For example, in FIG. 11, when DPS is performed, the UE receivesdata from eNB 1 or eNB 2. In this case, the UE receives the transmissioneNB through a DCI field. In the embodiment of FIG. 11, although thereare two eNBs, a maximum of 3 eNBs per UE can perform cooperativecommunication in a current LTE system, and thus, 2-bit field may beadded to DCI to transmit the transmission eNB information. In addition,when the number of eNBs that perform cooperative communication isincreased, a field corresponding to the increased number may be added toDCI to transmit the transmission eNB information.

Table 3 below shows an example of the aforementioned 2-bit field. The2-bit field is defined as a CSI process index or a CSI-RS index. Forexample, when the 2-bit field is configured as ‘00’, a UE can know thatdata is received through a DL channel measured using CSI-RS 0.

TABLE 3 2 bit Alt 1, CS1 Alt 2, CSI-RS DCI field process index index 00CSI process 0 CSI-RS 0 01 CSI process 1 CSI-RS 1 10 CSI process 2 CSI-RS2 11 Reserved reserved

The aforementioned transmission eNB information may be transmitted byadding a new field to DCI or using a reserved bit, use of which is notdefined, among fields defined in legacy DCI. For example, some statesreserved in 3-bit CIF field defined for CA may be defined as a CSIprocess index or a CSI-RS index as shown in Table 3 above.

The UE recognizes ZP CSI-RS information of eNB that actually transmitsdata based on ZP CSI-RS information per eNB and transmission eNBinformation, assumes that data is not mapped in a corresponding ZPCSI-RS resource element (RE), and performs data demodulation.

When the UE performs rate matching using the aforementioned method, theUE assumes that data is mapped in IMR present outside ZP CSI-RS of eNBthat transmits data among configured IMRs and performs datademodulation. That is, in the case of IMR resource contained in the ZPCSI-RS RE of eNB that actually transmits data, the UE assumes that datais not mapped in the corresponding IMR and performs data demodulation.On the other hand, in the case of IMR resource that is not contained inthe ZP CSI-RS RE of eNB that actually transmits data, the UE assumesthat data is mapped in the corresponding IMR and performs datademodulation.

For example, in FIG. 11, when the transmission eNB information indicateseNB 1, the UE assumes that data is not mapped in IMR 0 and IMR 2 andperforms data demodulation. In addition, the UE assumes that data ismapped in IMR 1 and performs data demodulation. On the other hand, whenthe transmission eNB information indicates eNB 2, the UE assumes thatdata is not mapped in IMR 1 and IMR 2 and performs data demodulation. Inaddition, the UE assumes that data is mapped in IMR 0 and performs datademodulation.

Through the aforementioned method, the UE may perform interferencemeasurement using an RE to which data is mapped in IMR. That is, whenresource configured as IMR is not further configured as ZP CSI-RS, theUE further determines that PDSCH is mapped to the correspondingresource. Upon receiving PDSCH through IMR that is not configured as ZPCSI-RS, the UE considers all reception signal including the PDSCHreceives for interference measurement in the corresponding IMR asinterference signals. In addition, the UE determines that a signal forUE, for receiving PDSCH, is present in the corresponding IMR.

At least one eNB performs muting in a CoMP measurement set, and thus,IMR needs to be present in a union of ZP CSI-RS REs of each eNB. The UEis not expected that IMR is configured not to completely overlap withone of the ZP CSI-RS REs. For example, when two 2 eNBs perform a CoMPoperation, the UE is configured by two ZP CSI-RSs. In this case, IMR ispresent in a union of two ZP CSI-RS REs.

The aforementioned IMR application method has been described in terms ofa network for convenience of description. That is, the aforementionedIMR application method has been described in that ZP CSI-RS isconfigured to each eNB that participates in CoMP and indicates eNB thatactually transmits data among eNBs.

From a UE point of view, the UE distinguishes eNBs that participate inCoMP based on configured CSI-RSs. For example, in FIG. 11, the UEdistinguishes eNB 1 and eNB 2 through two CSI-RS (i.e., CSI-RS 0 andCSI-RS 1) configured for the UE. Thus, from a UE point of view, anoperation for configuring ZP CSI-RS per eNB refers to an operation forconfiguring ZP CSI-RS per CSI-RS. In addition, an operation forindicating eNB that actually transmits data refers to an operation forindicating a DL channel of CSI-RS, from which data is actuallytransmitted, from a UE point of view. Accordingly, the UE is configuredby ZP CSI-RS information per CSI-RS from a network and is informed of aDL channel of CSI-RS, from which data is actually transmitted. The UErecognizes ZP CSI-RS of eNB that actually transmits data based on thetwo pieces of information, assumes that data is not mapped incorresponding ZP CSI-RS RE, and performs data demodulation.

In addition, the UE receives a plurality of CSI-RS configurations andreceives ZP CSI-RS configuration per CSI-RS. That is, one CSI-RS and oneZP CSI-RS resource are one to one connected. Furthermore, the UE isallocated a plurality of IMRs.

Upon receiving data through PDSCH allocated by DCI includingtransmission eNB information, the UE assumes that data is not mapped toall plural indicated CSI-RS resources but assumes that data is notmapped to only ZP CSI-RS resource corresponding to CSI-RS indicated bythe transmission eNB information with respect to the ZP CSI-RS resource.That is, the UE assumes that data is mapped to resource that is notincluded in ZP CSI-RS resource corresponding to CSI-RS indicated bytransmission eNB information with respect to IMR.

Upon receiving data through PDSCH allocated by DCI (e.g., DCI format 1Athat does not include transmission eNB information) that does notinclude transmission eNB indication information, the UE assumes thatdata is not mapped to all plural CSI-RS resources but assumes that datais not mapped to only specific resource, for a representative example,first ZP CSI-RS resource (with a lowest index) with respect to ZP CSI-RSresources. That is, the UE assumes that data is mapped to resource thatis not included in ZP CSI-RS resource with a lowest index with respectto IMR.

In a different method, upon receiving data through PDSCH allocated byDCI that does not include transmission eNB information, that is, DCI 1A,the UE may assume that data is not mapped to all plural indicated CSI-RSresources and ZP CSI-RS resources. In addition, the UE assumes that datais mapped to resource that is not included in ZP CSI-RS resource withrespect to IMR.

Since CSI-RS is RS that is actually transmitted with transmission powerand may be referred to as non-zero power (NZP) CSI-RS.

FIG. 12 is a flowchart illustrating a method for transceiving CSIaccording to a first embodiment of the present invention.

First, a UE receives, from serving cell, first resource configurationinformation for a CSI-RS (Channel-State Information-Reference Signal)and second resource configuration information for interferencemeasurement (S1210). Herein, the first resource configurationinformation may indicates the CSI-RS configuration information, and thesecond resource configuration information may indicates the IMR.

Then, the UE calculates CSI using the first resource configurationinformation and the second resource configuration information, the CSIbeing for one or more base stations (BSs) among a plurality of BSsparticipating the COMP (S1230).

Then, the UE could report the calculated CSI to serving cell (S1250).

Herein, the IMR according to the second resource configurationinformation may exist in a union of zero-power CSI-RS resources of eachof the plurality of BSs participating COMP. That is, the base stationmay configure the IMR within a union of zero-power CSI-RS resourcesconfigured to the UE.

Second Embodiment

Although the aforementioned first embodiment relates to PDSCH datamapping, when a UE receives enhance PDCCH (EPDCCH), the same method maybe extensively applied to DCI to RE mapping of EPDCCH.

In an LTE system, some regions of PDSCH may be indicated as EPDCCH andthe corresponding resource may be used to transmit control information.As illustrated in FIG. 13, the EPDCCH refers to enhanced PDCCH as acontrol channel transmitted in a PDSCH region instead of a legacy PDCCH.In FIG. 13, frequency resources used for EPDCCH are consecutivelyarranged, which is purely exemplary. That is, in order to acquirefrequency diversity, EPDCCH may be transmitted using spaced frequencyresources.

A BS may indicate a plurality of EPDCCH sets to one UE. Here, the EPDCCHset refers to a set of PRBs in which a series of EPDCCH blind decodingcandidates are present. PRBs included in the EPDCCH set may be given byhigher-layer signaling such as RRC signaling, etc. The UE assumes thateach candidate uses resources of the EPDCCH set to which thecorresponding candidate belongs upon attempting to detect a specificblind decoding candidate. In addition, the BS may configure variousdedicated properties for the respective EPDCCH sets. For example,whether an EPDCCH candidate uses localized transmission or distributedtransmission, a parameter used for HARQ ACK when a candidate thatbelongs to each EPDCCH set uses DL assignment, etc. may be configured.

When the UE is indicated by multiple EPDCCH sets from the BS by RRC, theUE configures the EPDCCH sets as a search space (SS) for decoding DCIand attempts blind decoding with respect to various aggregation levels.Each set may be indicated as multiple PRBs and other sets and some PRBsmay overlap.

In this case, other adjacent eNBs as well as a serving eNB may performEPDCCH transmission to the UE. For example, the following cases may beconsidered. As a first case, transmission of EPDCCH sets may beperformed by different eNBs. As a second case, EPDCCH transmission inPRBs in EPDCCH SS may be performed by different eNBs. As a third case,EPDCCH transmission in EPDCCH DMRS ports may be performed by differenteNBs. Hereinafter, each case will be described in detail, and DCI to REmapping of EPDCCH will be proposed.

As the first case, transmission of EPDCCH sets may be performed bydifferent eNBs. In this case, the BS may signal EPDCCH transmission eNBinformation of each set to the UE by higher layer signaling such as RRC,etc. The EPDCCH transmission eNB information is a CSI-RS index of EPDCCHtransmission eNB. Upon performing blinding decoding on DCI of each setfrom the information, the UE assumes that DCI is not mapped in ZP CSI-RSRE of EPDCCH transmission eNB of the corresponding set and performsblinding decoding. That is, the UE assumes that DCI is not mapped in ZPCSI-RS that is one to one connected to CSI-RS of the corresponding setand performs blind decoding. The UE assumes that DCI is mapped withrespect to IMR present outside the ZP CSI-RS and performs blinddecoding.

For example, in FIG. 11, two EPDCCH sets, that is, set 0 and set 1 areconfigured for the UE, eNB 1 transmits control information in set 0, andeNB 2 transmits control information in set 1. The UE is configured byCSI-RS 0 with respect to set 0 and is configured by CSI-RS 1 withrespect to set 1 from the BS by RRC. Upon performing blinding decodingon set 0, the UE assumes that DCI is not mapped in ZP CSI-RS that is oneto one connected to CSI-RS 0 and performs blind decoding. On the otherhand, upon performing blinding decoding on set 1, the UE assumes thatDCI is not mapped in ZP CSI-RS that is one to one connected to CSI-RS 1and performs blind decoding. The UE assumes that DCI is mapped in IMRpresent outside the corresponding ZP CSI-RS and performs blind decoding.

An index of CSI-RS connected to each EPDCCH set may be indicateddirectly through a field in an EPDCCH set configuration message.

In the case of direct indication, one EPDCCH set may be connected to twoor more CSI-RSs. In this case, the direct indication can be effectivelyused for an operation for simultaneously transmitting the same EPDCCH intwo or more eNBs.

As another example of direct indication, for easy channel estimation onEPDCCH by the UE, the BS may determine CSI-RS having the same long termcharacteristics such as Doppler spread or frequency offset as EPDCCH DMRS as a higher-layer signal. The CSI-RS may be, for example,quasi-colocated (QC) CSI-RS to be assumed to be transmitted at the sameposition. In this case, the higher-layer signal may be reused such thatDCI mapping is not performed on specific EPDCCH and QC CSI-RS, and ZPCSI-RS carried thereon.

In addition, an index of CSI-RS connected to each EPDCCH set may beindirectly indicated. For example, EPDCCH set 0 may be automaticallyconnected to CSI-RS 0 and EPDCCH set 1 may be automatically connected toCSI-RS 1.

As the second case, EPDCCH transmission in PRBs in EPDCCH SS may beperformed by different eNBs. In this case, the BS may inform the UE ofEPDCCH transmission eNB information of each PRB by higher-layersignaling such as RRC, etc. The EPDCCH transmission eNB information is aCSI-RS index of the EPDCCH transmission eNB. Upon performing blinddecoding on each PRB from the information, the UE assumes that DCI isnot mapped in ZP CSI-RS RE of EPDCCH transmission eNB of thecorresponding PRB and performs blind decoding. That is, the UE assumesthat DCI is not mapped in ZP CSI-RS that is one to one connected toCSI-RS of the corresponding PRB and performs blind decoding. The UEassumes that DCI is mapped in IMRs present outside the ZP CSI-RS andperforms blind decoding.

For example, in FIG. 11, two PRBs in EPDCCH SS, that is, PRB 0 and PRB 1are configured for the UE, eNB 1 transmits control information in PRB 0,and eNB 2 transmits control information in PRB 1. The UE is configuredby CSI-RS 0 with respect to PRB 0 and is configured by CSI-RS 1 withrespect to PRB 1 from the BS by RRC. Upon performing blinding decodingon PRB 0, the UE assumes that DCI is not mapped in ZP CSI-RS that is oneto one connected to CSI-RS 0 and performs blind decoding. On the otherhand, upon performing blinding decoding on PRB 1, the UE assumes thatDCI is not mapped in ZP CSI-RS that is one to one connected to CSI-RS 1and performs blind decoding. The UE assumes that DCI is mapped in IMRpresent outside the corresponding ZP CSI-RS and performs blind decoding.

In addition, when EPDCCH transmission of PRBs in EPDCCH SS is performedby different eNBs, DCI to RE mapping may be performed using thefollowing method. Upon performing blind decoding in each PRB, the UEsearches for CSI-RS having the same CSI-RS scrambling ID as scramblingID of DMRS allocated to the corresponding PRB among configured multipleCSI-RSs. Then, the UE assumes that DCI is not mapped in ZP CSI-RS thatis one to one connected to the CSI-RS and performs blind decoding.

In addition, the BS and the UE may search for CSI-RS using a mappingtable between CSI-RS scrambling ID and scrambling ID of predefined DMRS.The mapping table may be indicated to the UE from the BS by RRC. Then,the UE assumes that DCI is not mapped in ZP CSI-RS that is one to oneconnected to the CSI-RS and performs blind decoding.

As the third case, EPDCCH transmission in EPDCCH DMRS ports may beperformed by different eNBs. In this case, the BS may inform the UE ofEPDCCH transmission eNB information of each DMRS port by higher-layersignaling such as RRC, etc. The EPDCCH transmission eNB information is aCSI-RS index of the EPDCCH transmission eNB. Upon performing blinddecoding on each DMRS port from the information, the UE assumes that DCIis not mapped in ZP CSI-RS RE of EPDCCH transmission eNB of thecorresponding DMRS port and performs blind decoding. That is, the UEassumes that DCI is not mapped in ZP CSI-RS that is one to one connectedto CSI-RS of the corresponding DMRS port and performs blind decoding.The UE assumes that DCI is mapped in IMRs present outside the ZP CSI-RSand performs blind decoding.

For example, in FIG. 11, the UE may receive EPDCCH through DMRS port 7and DMRS port 9. In this case, eNB 1 transmits control informationthrough DMRS port 7 and eNB 2 transmits control information through DMRSport 9. The UE is configured by CSI-RS 0 with respect to DMRS port 7 andis configured by CSI-RS 1 with respect to DMRS port 9 from the BS byRRC. Upon performing blind decoding on DMRS port 7, the UE assumes thatDCI is not mapped in ZP CSI-RS that is one to one connected to CSI-RS 0and performs blind decoding. On the other hand, upon performing blinddecoding on DMRS port 9, the UE assumes that DCI is not mapped in ZPCSI-RS that is one to one connected to CSI-RS 1 and performs blinddecoding. The UE assumes that DCI is mapped in IMRs present outside thecorresponding ZP CSI-RS and performs blind decoding.

Thus far, the DCI to RE mapping method of EPDCCH has been described interms of the three cases in which a plurality of eNBs transmits EPDCCHto the UE. Briefly, with respect to all eNBs that can transmit EPDCCH tothe UE, DCI to RE mapping may be determined using a union of ZP CSI-RSsof each eNB. That is, the UE receives union information of ZP CSI-RSs ofthe eNB from a network and assumes that DCI is not mapped with respectto all configured ZP CSI-RSs during blind decoding of the EPDCCH.

Third Embodiment

In the aforementioned method, in order to determine whether data of IMRis mapped or whether DCI of IMR is mapped, ZP CSI-RS information andtransmission eNB information per eNB are transmitted. Thus, the UE canindirectly recognize whether data/DCI is mapped in IMR, from ZP CSI-RSinformation of data transmission eNB. In addition, as another method,information about whether data of IMR is mapped or whether DCI of IMR ismapped may be received directly from DCI as follows.

For example, as shown in Table 4 below, 3-bit field may be added to DCand whether data is mapped may be signaled to the UE. The UE receivesinformation of Table 4 through the DCI and assumes that data is notmapped in IMR configured by muting. The UE assumes that data is mappedin IMR configured by data transmission.

TABLE 4 New DCI field IMR 0 IMR 1 IMR 2 000 Mute Mute Mute 001 Mute MuteData 010 Mute Data Mute 011 Data Mute Mute 100 Mute Data Data 101 DataMute Data 110 Data Data Mute 111 (reserved)

For example, in FIG. 11, upon receiving data from eNB 1, the UE receives‘010’ from the BS through the DCI field. The UE assumes that data ismapped in IMR 1, does not assume that data is mapped in IMR 0 and IMR 2,and performs data demodulation. On the other hand, upon receiving datafrom eNB 2, the UE receives ‘011’ from the BS through the DCI field. TheUE assumes that data is mapped with respect to IMR 0, does not assumethat data is mapped with respect to IMR 1 and IMR 2, and performs datademodulation.

In addition, as shown in Table 4 above, a new field may not be added andwhether data of IMR is mapped may be determined using initial valueinformation of DMRS sequence in DCI. For example, when the initial valuecan be configured by 0 and 1 and the UE is set by 0, the UE assumes thatdata is mapped with respect to only IMR 0 among configured IMRs. The UEassumes that data is mapped with respect to only IMR 1 among configuredIMRs. In the aforementioned example, the initial value is limited to 1bit. However, according to an available bit number, more data mappingIMRs may be configured for the UE.

As described above, upon receiving information about whether data of IMRis mapped or whether DCI of IMR is mapped directly through DCI, the UEreceives one piece of ZP CSI-RS information from the BS and determineswhether data/DCI is mapped with respect to the remaining resourcesexcept for the IMR. The one piece of ZP CSI-RS information indicates aunion of ZP CSI-RSs of each eNB. For example, in FIG. 11, when ZP CSI-RSof eNB 1 is allocated to resources 1, 2, and 3 and when ZP CSI-RS of eNB2 is allocated to resources 3, 4, and 5, the UE recognizes that ZPCSI-RS is allocated to resources 1, 2, 3, 4, and 5 through the one pieceof ZP CSI-RS information and assumes that data/DCI is not mapped in thecorresponding resource.

FIG. 14 is a diagram illustrating a BS and a UE to which an embodimentof the present invention can be applicable.

When a relay is included in a wireless communication system,communication in backhaul link is performed between the BS and the relayand communication in access link is performed between the relay and theUE. Accordingly, the BS and UE illustrated in FIG. 13 can be replaced bya relay according to a situation.

Referring to FIG. 14, a wireless communication system includes a BS 1410and a UE 1420. The BS 1410 includes a processor 1413, a memory 1414, anda radio frequency (RF) unit 1411 and 1412. The processor 1413 may beconfigured to embody procedures and/or methods proposed according to thepresent invention. The memory 1414 is connected to the processor 1413and stores various information related to an operation of the processor1413. The RF unit 1411 and 1412 is connected to the processor 1413 andtransmits/receives a radio signal. The UE 1420 includes a processor1423, a memory 1424, and an RF unit 1421 and 1422. The processor 1423may be configured to embody procedure and/or methods proposed accordingto the present invention. The memory 1424 is connected to the processor1423 and stores various information related to an operation of theprocessor 1423. The RF unit 1421 and 1422 is connected to the processor1423 and transmits/receives a radio signal. The BS 1410 and/or the UE1420 may have a single antenna or a multiple antenna. The aforementionedembodiments of the present invention described are combinations ofelements and features of the present invention. The elements or featuresmay be considered selective unless otherwise mentioned. Each element orfeature may be practiced without being combined with other elements orfeatures. Further, an embodiment of the present invention may beconstructed by combining parts of the elements and/or features.Operation orders described in embodiments of the present invention maybe rearranged. Some constructions of any one embodiment may be includedin another embodiment and may be replaced with correspondingconstructions of another embodiment. It is obvious to those skilled inthe art that claims that are not explicitly cited in each other in theappended claims may be presented in combination as an embodiment of thepresent invention or included as a new claim by a subsequent amendmentafter the application is filed.

In the embodiments of the present invention, a specific operationdescribed as being performed by the BS may be performed by an upper nodeof the BS. Namely, it is apparent that, in a network comprised of aplurality of network nodes including a BS, various operations performedfor communication with a UE may be performed by the BS, or network nodesother than the BS. The term ‘BS’ may be replaced with a fixed station, aNode B, an eNode B (eNB), an access point, etc.

The embodiments according to the present invention can be implemented byvarious means, for example, hardware, firmware, software, or combinationthereof. In a hardware configuration, the embodiments of the presentinvention may be implemented by one or more application specificintegrated circuits (ASICs), digital signal processors (DSPs), digitalsignal processing devices (DSPDs), programmable logic devices (PLDs),field programmable gate arrays (FPGAs), processors, controllers,microcontrollers, microprocessors, etc.

In a firmware or software configuration, the embodiments of the presentinvention can be implemented by a type of a module, a procedure, or afunction, which performs functions or operations described above.Software code may be stored in a memory unit and then may be executed bya processor.

The memory unit may be located inside or outside the processor totransmit and receive data to and from the processor through variousmeans which are well known.

The detailed description of the preferred embodiments of the presentinvention is given to enable those skilled in the art to realize andimplement the present invention. While the present invention has beendescribed referring to the preferred embodiments of the presentinvention, those skilled in the art will appreciate that manymodifications and changes can be made to the present invention withoutdeparting from the spirit and essential characteristics of the presentinvention. For example, the structures of the above-describedembodiments of the present invention can be used in combination. Theabove embodiments are therefore to be construed in all aspects asillustrative and not restrictive. Therefore, the present inventionintends not to limit the embodiments disclosed herein but to give abroadest range matching the principles and new features disclosedherein.

Those skilled in the art will appreciate that the present invention maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent invention. The above embodiments are therefore to be construedin all aspects as illustrative and not restrictive. The scope of theinvention should be determined by the appended claims and their legalequivalents, not by the above description, and all changes coming withinthe meaning and equivalency range of the appended claims are intended tobe embraced therein. Therefore, the present invention intends not tolimit the embodiments disclosed herein but to give a broadest rangematching the principles and new features disclosed herein. It is obviousto those skilled in the art that claims that are not explicitly cited ineach other in the appended claims may be presented in combination as anembodiment of the present invention or included as a new claim by asubsequent amendment after the application is filed.

INDUSTRIAL APPLICABILITY

The above-described embodiments of the present invention can be appliedto a wireless communication system such as a user equipment (UE), arelay, a base station (BS), etc.

1. A method for reporting channel state information (CSI) by a userequipment (UE), the method comprising: receiving information on multipleCSI processes, each of the multiple CSI processes associated with aCSI-reference signal (CSI-RS) resource and an interference measurementresource (IMR); receiving downlink control channel (DCI) via a physicaldownlink control channel (PDCCH); performing interference measurementbased on an IMR of a predetermined CSI process among the multiple CSIprocesses; and reporting CSI based on the interference measurement,wherein the IMR of the predetermined CSI process used for theinterference measurement is specified by a field of the DCI received viathe PDCCH.
 2. The method of claim 1, wherein a size of the fieldspecifying the IMR of the predetermined CSI process corresponds to2-bit.
 3. The method of claim 1, wherein the field of the DCI indicatesa CSI-RS for signal measurement.
 4. The method of claim 1, furthercomprising: receiving multiple zero-power CSI-RS (ZP CSI-RS)configurations for multiple transmission points configured to performcoordinated multi-point (CoMP) operation.
 5. The method of claim 4,wherein each of the multiple ZP CSI-RS configurations includes a periodof a ZP CSI-RS, subframe offset and ZP CSI-RS resource information. 6.The method of claim 1, wherein the IMR of the predetermined CSI processis included in one of the multiple ZP CSI-RS configurations.
 7. Themethod of claim 1, wherein the CSI includes a channel quality indicator(CQI) computed based on the interference measurement.
 8. A method forreceiving channel state information (CSI) by a base station, the methodcomprising: transmitting information on multiple CSI processes, each ofthe multiple CSI processes associated with a CSI-reference signal(CSI-RS) resource and an interference measurement resource (IMR);transmitting downlink control channel (DCI) via a physical downlinkcontrol channel (PDCCH); and receiving CSI based on interferencemeasurement, the interference measurement performed based on an IMR of apredetermined CSI process among the multiple CSI processes, wherein theIMR of the predetermined CSI process used for the interferencemeasurement is specified by a field of the DCI transmitted via thePDCCH.
 9. The method of claim 8, wherein a size of the field specifyingthe IMR of the predetermined CSI process corresponds to 2-bit.
 10. Themethod of claim 8, wherein the field of the DCI indicates a CSI-RS forsignal measurement.
 11. The method of claim 8, further comprising:transmitting multiple zero-power CSI-RS (ZP CSI-RS) configurations formultiple transmission points configured to perform coordinatedmulti-point (CoMP) operation.
 12. The method of claim 11, wherein eachof the multiple ZP CSI-RS configurations includes a period of a ZPCSI-RS, subframe offset and ZP CSI-RS resource information.
 13. Themethod of claim 8, wherein the IMR of the predetermined CSI process isincluded in one of the multiple ZP CSI-RS configurations.
 14. The methodof claim 8, wherein the CSI includes a channel quality indicator (CQI)computed based on the interference measurement
 15. A user equipment (UE)for reporting channel state information (CSI), the UE comprising: areceiver configured to receive information on multiple CSI processes,each of the multiple CSI processes associated with a CSI-referencesignal (CSI-RS) resource and an interference measurement resource (IMR),and to receive downlink control channel (DCI) via a physical downlinkcontrol channel (PDCCH); and a processor configured to performinterference measurement based on an IMR of a predetermined CSI processamong the multiple CSI processes and to report CSI based on theinterference measurement, wherein the IMR of the predetermined CSIprocess used for the interference measurement is specified by a field ofthe DCI received via the PDCCH.
 16. A base station for receiving channelstate information (CSI), the BS comprising: a transmitter configured totransmit information on multiple CSI processes, each of the multiple CSIprocesses associated with a CSI-reference signal (CSI-RS) resource andan interference measurement resource (IMR), and to transmit downlinkcontrol channel (DCI) via a physical downlink control channel (PDCCH); areceiver configured to receive CSI based on interference measurement,the interference measurement performed based on an IMR of apredetermined CSI process among the multiple CSI processes; and aprocessor configured to control the receiver and the transmitter,wherein the IMR of the predetermined CSI process used for theinterference measurement is specified by a field of the DCI transmittedvia the PDCCH.